This invention relates to an acoustic lining for ducts for attenuating noise propagating through a fluid flowing within the duct; and, more particularly, to such a lining for use in the intake duct of a combustion turbine engine for attenuating the noise generated by the compressor of the engine.
A wide range of apparatus and methods are known for attenuating noise associated with flowing fluids such as air in a duct. One such method is to line the duct with a sound-absorbing bulk material such as fiber glass batting or cellular plastic foam. Another such method is to provide a sound absorbing structure including a perforate sheet bounding the air flow path on one side and on the other side bounding one or more sound attenuating cells. The sound attentuating cells may be partially or completely filled with a sound-absorbing bulk material. U.S. Pat. Nos. 3,033,307; 3,221,500 and 3,511,336 along with British Pat. No. 479,823 illustrate conventional apparatus and methods of attenuating noise in flowing fluids.
However, when conventional noise attentuating techniques are applied to the intake duct of a combustion turbine engine many problems and deficiencies become apparent. For example, bulk absorber materials are generally fibrous in nature and are fluid permeable. Thus, liquids such as water, fuel, and lubricating oil, which are frequently present in the intake duct of a combustion turbine engine, may soak into the absorber material reducing its noise attenuating capability and creating excess weight. Further, fuel and lubricating oil present a fire hazard. Moreover, dust, dirt and other particulate contaminants present in the air flowing into the engine are attracted to the bulk absorber material wetted with liquid. Such contaminants, if allowed to accumulate, may partially obstruct the engine intake duct and in extreme cases may slough off in aggregate chunks large enough to damage the engine. Moreover, bulk absorber materials generally do not have sufficient strength to withstand the rigours of use in a turbine engine. Consequently, such bulk materials must ordinarly be supported by a sheet metal structure. However, the sheet metal support structure must be at least partially perforate to allow noise to penetrate to the bulk absorber material. Consequently, liquids may also penetrate to soak into the bulk material. As a result, in some applications it has been proposed to employ the sheet metal support structure alone without the bulk absorber material. Purportedly, the sheet metal structure with perforations leading to empty cells, which would normally house the bulk material, will attenuate noise. However, the shortcomings of such a simplistic solution to the problem have been recognized. Therefore, another conventional proposal has been to provide a rather elaborate sheet metal wall structure defining baffles, moats and drains for liquid control between the air flow path and the bulk absorber material. Of course, such a structure is somewhat complex and expensive to produce and cannot entirely eliminate the problem of liquid soaking into the bulk absorption material. U.S. Pat. No. 4,035,535 illustrates a recent example of this latter approach to the problem outlined above.
In addition to all of the deficiencies outlined above, conventional sound attenuating structures have a further deficiency which until now has been overshadowed by the need to provide satisfactory sound attenuation while avoiding liquid absorption. This further deficiency stems from the perforate sheet metal which conventionally confronts the air flow. The problem is that the multitude of perforations interrupt the surface of the sheet metal so that they interrupt also the smooth flow of air along the sheet metal. The perforate sheet metal does not provide a desirable aerodynamic surface for the air flow. In other words, the perforations, although necessary, are undesirable because they generate turbulence and friction in the flowing fluid and a concomitant undesirable fluid pressure drop.
In view of the deficiencies of conventional noise attenuating apparatus, it is an object of this invention to provide duct lining method and apparatus which satisfactorily attenuates noise without the use of a bulk sound-absorbing material.
Another object is to provide duct lining which presents a substantially smooth outer surface to the fluid flowing in the duct.
Yet another object is to provide such a duct lining which is particularly useful in combustion turbine engines.
Still another object for this invention is to provide duct lining having an outer surface which is impermeable to liquids at atmospheric pressure.
Another object is to provide a kit for retrofitting duct lining according to this invention to combustion turbine engines which may benefit from the improved noise attenuation offered by this invention.
In summary, this invention provides a duct lining having a first layer of air-permeable wire cloth defining a substantially smooth outer surface confronting the flowing fluid in a duct. The wire cloth is secured to a second support layer of perforate sheet metal. The second layer is secured to and spans the openings of a honeycomb structure defining resonator cells which are tuned to effect the greatest attentuation of the band of noise which is most objectionable.
Other objects and advantages of this invention will appear in light of the following detailed description of preferred embodiment of the invention.